alter paths for boost libs in Blender's CMake to use my system boost
Hm, it is strange it downloaded boost.

If you are looking to change the CMake file, then go to line

find_package(Boost bla bla COMPONENTS ${__boost_packages})


Before that line, insert:

SET(BOOST_ROOT "path/to/boost/")
SET(Boost_NO_SYSTEM_PATHS YES)


With this, it should look for boost only in the specified BOOST_ROOT directory. You might need to move these commands to the upper-most levels of the if's.